Infrared radiation communication has been taking root in the market as transmission and reception modules get smaller, higher in performance, and speedier.
FIG. 7 is a block diagram illustrating a receiving circuit of a conventional infrared radiation communication system.
In this receiving circuit, as shown in FIG. 7, a photocurrent signal is inputted to a photodiode chip (PD) 101. Then, the photocurrent signal is amplified through amplifies (amp) 102 and 103 provided in an integrated receiving chip, thereby generating an amplified signal (Amp_out). The amplified signal is pulse-shaped through a comparator circuit (Comp) 104 by using a threshold value (Thresh). In this way, a comparator signal (Comp_out) is generated.
In accordance with the comparator signal, a one shot pulse generating circuit 105 generates a one shot pulse signal (OS_out). The one shot pulse signal is inverted through an inverter 106, and outputted as an output signal (VO) from an output terminal 107. The output signal (VO) is transmitted to a controller LSI and processed therein.
FIG. 8 is a waveform chart illustrating simplified waveforms of the optical signal (input signal) outputted to the receiving circuit, the amplified signal (Amp_out), the threshold value (Thresh), the comparator signal (Comp_out), the one shot pulse signal (OS_out), and the output signal (VO).
The one shot pulse generating circuit 105 shown in FIG. 7 is a circuit for generating a one shot pulse signal of a predetermined pulse width in accordance with the comparator signal. Depending on various receiving conditions, the waveform of the output signal can be distorted. The one shot pulse generating circuit 105 prevents the distortion.
The following publication 1 is an example of this art.
Publication 1: Japanese Publication for Unexamined Patent Application, Tokukai 2002-232271 (publication date: Aug. 16, 2002).
Recently, however, the receiving circuit (receiving module device) has been getting smaller. Therefore, the distance between the receiving terminal (which receives an input signal inputted from outside) and the output terminal is physically short. As a result, the input signal and the output signal are easily coupled to each other, and the influence of coupling appears in the output signal.
If the circuit is small, it is no longer possible to ignore capacitance coupling (C_cup) that occurs between the output terminal 107 and input sections of such members as the photodiode chip 101 and the amplifier 102 of FIG. 7 through package resin or the like. Due to the capacitance coupling, noises appear on the amplified signal (Amp_out) at the timing of, for example, voltage transition (voltage fluctuation) of the output signal VO. The noises distort the waveform of the amplified signal (Amp_out).
As shown in FIG. 9, the distortion of the amplified signal (Amp_out) causes unnecessary pulses in the comparator signal (Comp_out). Due to the pulses, a one shot pulse signal is generated. As a result, unnecessary pulses appear in the output signal VO and cause malfunctions.